Datasheet

0.991
92417
RT (k ) = = 244 k
400 (kHz)
W W
SW(shift)
8 6 A x 11 m 0.1 V 0.7 V
855 kHz
135 ns 60 V - 6 A x 92 m 0.7 V
W + +
æ ö
= ´ =
ç ÷
W +
è ø
f
SW(max skip)
1 5 A x 11 m 5 V 0.7 V
708 kHz
135ns 60 V - 5 A x 92 m 0.7 V
W + +
æ ö
= ´ =
ç ÷
W +
è ø
f
TPS54560
www.ti.com SLVSBN0A –MARCH 2013–REVISED MARCH 2014
The typical minimum on time, t
onmin
, is 135 ns for the TPS54560. For this example, the output voltage is 5 V and
the maximum input voltage is 60 V, which allows for a maximum switch frequency up to 708 kHz to avoid pulse
skipping from Equation 8. To ensure overcurrent runaway is not a concern during short circuits use Equation 9 to
determine the maximum switching frequency for frequency foldback protection. With a maximum input voltage of
60 V, assuming a diode voltage of 0.7 V, inductor resistance of 11 m Ω, switch resistance of 92 mΩ, a current
limit value of 6 A and short circuit output voltage of 0.1 V, the maximum switching frequency is 855 kHz.
For this design, a lower switching frequency of 400 kHz is chosen to operate comfortably below the calculated
maximums. To determine the timing resistance for a given switching frequency, use Equation 6 or the curve in
Figure 6. The switching frequency is set by resistor R
3
shown in Figure 37. For 400 kHz operation, the closest
standard value resistor is 243 kΩ.
(24)
(25)
(26)
9.2.2.2 Output Inductor Selection (L
O
)
To calculate the minimum value of the output inductor, use Equation 27.
K
IND
is a ratio that represents the amount of inductor ripple current relative to the maximum output current. The
inductor ripple current is filtered by the output capacitor. Therefore, choosing high inductor ripple currents
impacts the selection of the output capacitor since the output capacitor must have a ripple current rating equal to
or greater than the inductor ripple current. In general, the inductor ripple value is at the discretion of the designer,
however, the following guidelines may be used.
For designs using low ESR output capacitors such as ceramics, a value as high as K
IND
= 0.3 may be desirable.
When using higher ESR output capacitors, K
IND
= 0.2 yields better results. Since the inductor ripple current is
part of the current mode PWM control system, the inductor ripple current should always be greater than 150 mA
for stable PWM operation. In a wide input voltage regulator, it is best to choose relatively large inductor ripple
current. This provides sufficienct ripple current with the input voltage at the minimum.
For this design example, K
IND
= 0.3 and the inductor value is calculated to be 7.6 μH. The nearest standard value
is 7.2 μH. It is important that the RMS current and saturation current ratings of the inductor not be exceeded. The
RMS and peak inductor current can be found from Equation 29 and Equation 30. For this design, the RMS
inductor current is 5 A and the peak inductor current is 5.8 A. The chosen inductor is a WE 7447798720, which
has a saturation current rating of 7.9 A and an RMS current rating of 6 A.
As the equation set demonstrates, lower ripple currents will reduce the output voltage ripple of the regulator but
will require a larger value of inductance. Selecting higher ripple currents will increase the output voltage ripple of
the regulator but allow for a lower inductance value.
The current flowing through the inductor is the inductor ripple current plus the output current. During power up,
faults or transient load conditions, the inductor current can increase above the peak inductor current level
calculated above. In transient conditions, the inductor current can increase up to the switch current limit of the
device. For this reason, the most conservative design approach is to choose an inductor with a saturation current
rating equal to or greater than the switch current limit of the TPS54560 which is nominally 7.5 A.
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